1. Field of the Invention
The invention relates to supply voltage level detectors.
2. Description of the Related Art
FIG. 1 shows a conventional supply voltage level detector 100, for detecting the voltage level of a voltage source of a chip (VDD). The supply voltage level detector 100 comprises a bandgap reference voltage generator 102, a voltage source divider 104, and a comparator CMP. The bandgap reference voltage generator 102, is a basic chip component, and generates a bandgap reference voltage VBG. The bandgap reference voltage VBG has a constant value (generally about 1.25 volts) at all temperatures and for all voltage sources. The voltage source divider 104 comprises three transistors, M1, M2, and M3, and two resistors R1 and R2. After the bandgap reference voltage VBG is generated, the transistor M1 is turned on, the transistor M2 operates in a saturation region, the transistor M3 is turned on to generate a current through the resistors R1 and R2, and the voltage source divider 104 divides the voltage source VDD to generate a detection voltage Vdet. In the conventional supply voltage level detector 100, the area of the transistor M3 is greater than the transistor M2, thus, the transistor M3 is operated in a linear region and acts as a resistor. The on resistance of the transistor M3 is Ron. The detection voltage Vdet is generated by dividing the voltage source VDD by the on resistance Ron, and the resistors R1 and R2, wherein Vdet=VDD·R2/(Ron+R1+R2). The comparator CMP compares the detection voltage Vdet with the bandgap reference voltage VBG and generates a flag signal (Flag) indicating the comparison result. The chip determines if the voltage source VDD is available based on the flag signal (Flag).
The conventional supply voltage level detector 100 does not always function properly. When the voltage source of the chip is reset, the bandgap reference voltage generator 102 does not generate the bandgap reference voltage VBG until the voltage source VDD returns to a specific voltage level. The comparator CMP outputs an unreliable flag signal when the bandgap reference voltage VBG is not ready.
In some cases, the normal value of the voltage source VDD is 5 volts, and the specific voltage level is 2.5 volts. When the voltage source is reset, the bandgap reference voltage generator 102 does not output the bandgap reference voltage VBG until the supply voltage VDD returns to a level greater than the specific voltage level (2.5 volts). It can be seen that comparator CMP only outputs a reliable flag signal after the supply voltage VDD returns to the specific voltage level. Thus, a novel supply voltage level detector capable of reliable flag signal (Flag) output is desirable.